A lithographic printing plate precursor capable of forming an image by a heat source such as a heat-sensitive head, etc., and a lithographic printing plate precursor capable of forming an image by heat which is converted from irradiated light, are known in the lithographic printing technique. In particular, the latter lithographic printing plate precursor is promising as the lithographic printing plate precursor for computer to plate (CTP) technique capable of directly making a printing plate without necessitating a film by scanning highly directional active radiant rays such as laser rays in accordance with digitized image data.
As the support of a lithographic printing plate precursor capable of forming an image by the work of heat (called heat-sensitive type image recording), metals such as aluminum and polyethylene terephthalate (PET) are widely used.
When metals are used as the support of a lithographic printing plate precursor, since metals do not absorb water and a solvent, moreover, are excellent in strength, even if water and a solvent is used after that in a developing step, dimensional accuracy of the support does not deteriorate and the recorded image is correctly reproduced. Further, since metals are fundamentally excellent in strength, the obtained lithographic printing plate precursors have excellent press life in many cases.
However, since metals have a high heat-conductivity, even if it is tried to form an image by irradiation of light for image-recording, e.g., laser rays, and converting the light to heat, the heat is liable to be dissipated to a metal support and a satisfactory image is difficult to be formed. That is, the sensitivity of the lithographic printing plate precursor becomes very low, and staining due to ink is generated on the non-image area of a printed matter. Therefore, sufficient energy becomes necessary to form an image, for instance, by raising light output and delaying writing time, but if light output is increased, the manufacturing cost of a writing unit increases, as a result, the advantage of the manufacturers of the unit is lowered, and if writing time is delayed, plate-making takes longer time, as a result, the advantage of the printing press is lowered.
On the other hand, when PET is used as the support of a lithographic printing plate precursor, it has been known that since PET is comparatively low in heat conductivity as compared with metals, PET is very advantageous in the point of the minimum energy necessary for writing an image, i.e., sensitivity.
However, since PET absorbs water, although as small as 0.4% or so, when water and a solvent are used at printing, a support absorbs water and the dimension of a printing plate extends in some cases. Accordingly, in particular in four color printing, images are not be correctly reproduced (image disorder occurs) and in many cases unmarketable. There is another drawback that PET is lower in strength as compared with metals, hence the press life is inferior.
For the purpose of aiming at solving the above problem, a technique of providing a heat-insulating layer on the metal surface of the support of a lithographic printing plate precursor, and a hydrophilic hardening layer thereon is disclosed in WO 96/507727. However, the compatibility of sensitivity with press life could not be solved even by this technique.